PRESSURE VESSEL INSPECTIONS WALES
Both manufacturers and users should be familiar with the statutory requirements applying to compressed air systems and air receivers. These are principally contained in the Pressure Systems and Transportable Gas Containers Regulations 1989. Additional recommendations are given in the Approved Code of Practice Safety of pressure systems.
In premises where the air receivers must be cleaned and examined at regular intervals and for air receivers with a value of pressure times volume greater than 250 bar litres, the nature and frequency of the examination must be contained in a written scheme of examination. The written scheme of examination drawn up or certified by a competent person will normally specify a period between examinations of 24 to 48 months.
When connected to an air compressing plant a receiver should either be constructed to withstand the compressor’s maximum pressure safely, or be fitted with a safety valve and a pressure-reducing valve to prevent the receiver’s safe working pressure from being exceeded. When providing a safety valve and pressure gauge for a set of air receivers that are interconnected and collectively supplied through a single pipe, they may be treated as one receiver, provided that:
(a) any reducing valve or other suitable appliance is provided in the single supply pipe; and
(b) there is no means of isolating any interconnecting pipework.
The receiver should be fitted with a suitable safety valve complying with BS 6759, Part 2 or similar equivalent standard and adjusted so that air is allowed to escape as soon as the safe working pressure has been exceeded. It is essential that the valve is capable of discharging more air than the system can supply to the receiver.
The receiver should be fitted with a correct pressure gauge complying with BS 1780 or similar equivalent standard and which indicates pressure in bar, lbf/sqin, or other suitable units.
There should be a suitable appliance, either manual or automatic, for draining the receiver. It is recommended that manual drain valves should be full bore and straight through, to minimise any build-up of debris which could prevent tight shut off. Automatic drain valves should have adequate capacity for liquid discharge, be designed to minimise debris build-up and have a manual override to check performance. A strainer positioned immediately upstream of the valve will help prevent debris build- up. Protection against freezing of valves will be required in in low temperature conditions.
A receiver should have a suitable manhole, handhole or other means (as determined by the relevant standards, for example BS EN 286-1 Specification for simple unfired pressure vessels designed to contain air or nitrogen and BS 5169 Specification for fusion welded steel air receivers) to enable the interior to be thoroughly cleaned and inspected. Compressed air safety Page 12 of 50 Health and Safety Executive
A receiver should be clearly marked, in a conspicuous position, with its safe working pressure and other relevant information required by the Pressure Systems and Transportable Gas Containers Regulations 1989. The details should be clearly visible on the vessel or a plate attached to it.
A receiver should be readily distinguishable. This can be achieved by painting on identification markings or by affixing a manufacturer’s plate, giving the name of the manufacturer, serial number etc.
The main hazard is that the vessel may explode because its safe working pressure has been exceeded or because its ability to withstand pressure has been reduced through corrosion fatigue or internal coke fires for example.
General information and advice
All new air receivers should conform to BS EN 286-1 or BS 5169, where applicable, or to a standard providing equal integrity.
To determine the required capacity, the compressor output and demand pattern need to be taken into account. As a guide, this will be between six and ten times the free air output of the compressor at normal pressure (litres/second). Many users try to save on initial cost and make the mistake of buying a receiver that is too small.
The drain valve, safety valve, examination holes and manholes need to be accessible.
The scale of any gauges needs to be clearly visible.
On large compressor systems the air receiver should be fitted with a fusible plug conforming to BS 1123, Part 1. If a fire breaks out nearby, the plug will help to prevent an explosion by guarding against:
(a) the air temperature rising to the compressor lubricating oil flashpoint; or
(b) an explosion occurring below the designed working pressure because of lost integrity owing to heat.
Welded-on brackets are often used to mount compressor-motor combinations onto air receivers. Where this is the case, welding should be to the specification and approval of the air receiver manufacturer. It should not be carried out indiscriminately because the welder may unwittingly weaken the receiver. Mounting brackets should be designed and fitted in accordance with the applicable air receiver standard. It is good practice to weld mounting brackets onto intermediate reinforcing plates rather than directly to the air receiver.
Q. Do all air receivers need to be inspected? What is the calculation for deciding when an air receiver needs to comply with statutory regulations?
Within the Pressure Systems Safety Regulations 2000 the question is asked: Is the pressure x volume of the pressure vessel greater than 250 bar litres? If YES – then a written scheme and inspection certificate will need to be issued to comply with the legislation.
This calculation takes the pressure rated in bars and this is multiplied by the capacity of the tank in litres. This is commonly found on a plate on the receiver.
e.g. MWP 11 bar
50 litres capacity
In this example the maximum working pressure (MWP) is 11 bar multiplied by the 50 litre capacity, which gives a rating of 550 bar litres. 1 bar is equivalent to approximately 15 psi. As a general rule air receivers with a diameter in excess of 12 inches operating at 150psi will probably need to comply with the legislation.
The other components of the air compressor set (i.e. compressor and motor) do not need a statutory inspection. If an air compressor has no receiver it does not need an inspection. Hydrovane manufacture compressors with no receivers and so are exempt from the legislation.
Q. What is the difference between an air receiver and an air compressor? Do they both need an inspection?
An air compressor has three component parts:
- driving motor
- compressor – which compresses the air
- air receiver – which is a pressure vessel which stores the compressed air
The air receiver is the only part of the air compressor unit which needs a “statutory” examination. The air compressor and driving motor do not need an inspection – but should be regularly serviced/maintained by the user.
Some air compressors only have two components – driving motor and air compressor and the delivery of compressed air is immediate and the need to store air in a pressure vessel is not required. Because these compressors do not have air receiver they do not need examination.
The term “compressor” is also used for refrigeration compressors on fridges, cold stores, freezers and air conditioning systems. Refrigeration compressors only require examination under regulations where the driving motor exceeds 25kw. The majority of commercial cold stores operate using motors well below this limit.